In a conventional fuel supply system for an internal combustion engine, a high pressure pump is provided for pressurizing fuel. The fuel pressurized by the high pressure pump is supplied to a delivery pipe and accumulated therein. The high pressure fuel is then injected into respective cylinders of the engine via injectors connected to the delivery pipe.
The fuel supply system and the high pressure pump are disclosed, for example, in Japanese Patent Publication No. 2004-138062, and an International Patent Publication No. 2002-515565 (publication in Japanese). According to the above prior arts, a pressure relief valve is provided in a fuel supply system in order to avoid the following situation. Namely, when any malfunction occurs in controlling the high pressure pump, fuel pressure in the delivery pipe may become excessively high and the injectors become unable to perform fuel injection operation. And the delivery pipe may be damaged by such excessively high fuel pressure. The pressure relief valve is provided in a fuel return passage, which is connected between a high-pressure side fuel passage and a low-pressure side fuel passage. The high-pressure side fuel passage corresponds to a passage formed at a downstream side of a fuel discharge valve, which discharges a high pressure fuel pressurized by a fuel pressurizing chamber of the high pressure pump. The low-pressure side passage corresponds to a passage formed at an upstream side of the fuel discharge valve. The pressure relief valve is opened, when a differential pressure between the high-pressure side fuel passage and the fuel pressurizing chamber becomes higher than a predetermined pressure set by the pressure relief valve, so as to decrease the fuel pressure in the high-pressure side fuel passage.
According to the pressure relief valve (JP 2004-138062), a valve member is formed in a ball shape. Therefore, a force of flowing fuel (a dynamic pressure of the fuel) may not be sufficiently applied to the valve member when the pressure relief valve is opened. It would need a certain time period, during which the valve member is lifted from (separated from) a valve seat by a sufficient amount of stroke. It is, therefore, difficult to quickly decrease the fuel pressure in the high-pressure side fuel passage. Then, it may happen that the fuel pressure in the delivery pipe becomes higher than a fuel pressure for enabling the fuel injection operation by the injectors. In addition, the delivery pipe may be damaged.
According to the pressure relief valve, for example, as shown in FIG. 1 of JP 2002-515565, a valve member has a column shaped shaft portion extending from a forward end (a valve seat side) in an axial direction of a fuel return passage, a conical portion extending from a rear end of the column shaped shaft in a radial outward direction, and a guide portion extending from an outer periphery of the conical portion in the axial direction, the guide portion sliding on an inner surface of the fuel return passage. A flow restricting element is further provided at an upstream side of the valve seat for controlling fuel flow. According to the above structure, the dynamic pressure of the fuel may not be effectively applied to the conical portion at the valve opening time of the pressure relief valve. It would need a certain time period for sufficiently opening the pressure relief valve. Therefore, it may be difficult to quickly reduce the fuel pressure in the high-pressure side fuel passage.
In addition, according to the pressure relief valve, as shown in FIG. 3 of JP 2002-515565, a flow restricting portion is formed at the conical portion of the valve member in addition to the flow restricting element at the upstream side of the valve seat. According to such a structure, it would take a time for the valve member to be seated on the valve seat when the pressure relief valve is to be closed, because fuel pressure in a chamber between the flow restricting element at the upstream side of the valve seat and the flow restricting portion of the conical portion is applied to the conical portion of the valve member. In a case that the pressure relief valve does not surely close the fuel return passage, vibration may be generated in the fuel pressure of the delivery pipe. Furthermore, the pressure relief valve may be re-opened by pressure wave of the fuel, which is transmitted from the high-pressure side passage to the fuel return passage. Then, the fuel pressure in the delivery pipe may be drastically decreased. It may deteriorate the fuel injection from the injectors.